In line with the recent development in the information society, market needs have emerged for more compact magnetic disk drives offering higher densities and higher speeds. Perpendicular recording is suitable for these market needs. For a theory of operation involved thereof, the perpendicular recording system is suitable for achieving higher surface recording densities for the following reasons; specifically, the higher the linear recording density of a pattern recorded on a disk has, the smaller the demagnetizing field is for more stabilized magnetization; a leak component from a magnetic field generated from a write head is small in a track width direction; and the like. The perpendicular recording system also acts favorably on thermal decay characteristics of the disk. As compared with longitudinal magnetic recording medium, therefore, the perpendicular recording system imposes less stringent restrictions on development of medium. It can therefore be expected to realize even lower noise medium. From the foregoing advantages of the perpendicular recording system, there will be a major shift in magnetic disk drives to perpendicular recording in the near future.
A perpendicular recording magnetic head includes a read section and a write section, one stacked on top of the other. The read section includes a lower shield layer and an upper shield layer. The read section further includes a read element surrounded by the upper and lower shield layers and having a portion exposed to an air bearing surface. Types of the read element used include a giant magnetoresistive effect head, a tunneling giant magnetoresistive effect head having a large read output, and a CPP type giant magnetoresistive effect head applying current perpendicularly to a film surface. The write section forms a magnetic gap on the side of the air bearing surface. The write section includes a main magnetic pole piece and a sub-magnetic pole piece magnetically coupled to each other on a side opposite the air bearing surface. A coil is disposed between the main magnetic pole piece and the sub-magnetic pole piece. In perpendicular magnetic recording, a magnetic field component in a perpendicular direction from the main magnetic pole piece is used to record information. To achieve this purpose, a soft magnetic underlayer (SUL) is disposed on a lower portion of a recording layer of the disk. The SUL opposes the main magnetic pole piece, which allows a ferromagnetic field of a perpendicular component to be generated. A magnetic flux in the SUL is returned, for circulation, to a soft magnetic film of the magnetic head that serves as the sub-magnetic pole piece.
A higher linear recording density (BPI) and a higher track density (TPI) are required to achieve a high recording density. To improve BPI, it is typically necessary to improve magnetic field gradient from a write head and to improve resolution of a read head. To improve TPI, it is necessary to make a track width of the write head narrower. With the track width becoming narrower, however, there is a remarkable trend toward decreased head magnetic field intensity and an expanded effective track width that is recorded on the medium relative to an actual geometrical track width. Japanese Patent Publication No. 7-153013 discloses an arrangement, in which a yoke is disposed on either side of a main magnetic pole piece. The arrangement is intended to reduce a leak magnetic flux that represents the following problem. Specifically, in perpendicular recording, a closed magnetic circuit through which the magnetic flux flows is long. This decreases the amount of magnetic flux due to the leak magnetic flux. This, in turn, decreases a read/write output. U.S. Patent Publication No. 2002/0176214 and Japanese Patent Publication No. 2004-127480 disclose an arrangement, in which a side shield is disposed near the main magnetic pole piece in order to suppress expansion of the effective track width. Japanese Patent Publication No. 2006-277834 discloses a structure of the main magnetic pole piece having a side surface recessed inwardly.
As described above, the write head is required to have a narrow track width to achieve high recording densities. To achieve that end, it becomes necessary to narrow down the geometrical track width of the write head. Narrowing the geometrical track width, however, entails a decrease in the magnetic field generated from the write head, with which data is written in the medium, resulting in degraded writing performance. In the prior art using a head of a single magnetic pole piece structure for perpendicular recording, measures are taken to make up for a reduction in the magnetic field from the write head as a result of the narrowed track width. The measures include: shortening a flare point of the main magnetic pole piece; and, increasing the film thickness. The shorter flare point leads to degraded geometrical track width accuracy and an expanded distribution of the head magnetic field in the track width direction, which makes it difficult to achieve a magnetically narrow track. Increasing the film thickness, on the other hand, poses a problem in that part of the main magnetic pole piece protrudes, if provided with a skew angle, to a space over a track adjacent to a recorded track, so that a signal recorded therein is erased. Even in a magnetic head including a side shield, the presence of the side shield invites a reduction in the recording magnetic field, while an expansion of writing can be reduced. This makes it necessary, as in the structure having no side shields, to take such measures as shortening the flare point of the main magnetic pole piece and increasing the film thickness. In an arrangement having yokes on both sides of the main magnetic pole piece, the magnetic field distribution expands in the track width direction if there is a magnetic field leak directly from the yokes to the recording medium. This hampers the attempt to narrow the track width. The arrangement also poses a problem in that the magnetic field leaking to the recording medium erases the information in the adjacent tracks.